In an optical head device for reading information from an optical disk wherein a variety of high-density information is stored, the reading of information is performed by projecting a laser beam to produce a beam spot converging onto a arrow recording track and detecting a reflected beam therefrom.
FIG. 12 shows an example of the optical head device for reading information stored in a recording medium as mentioned above, in which a laser beam emitted from a laser beam source 1 passes through a diffraction grating 2 for producing a beam spot for tracking and is irradiated onto an optical disk 6 through a collimating lens 4 and an objective lens 5 after being reflected by a beam splitter 3. The beam reflected from the optical disk 6 is transmitted to the beam splitter 3 through the objective lens 5 and the collimating lens 4 and is incident upon a photodetector 8 through a plane-concave lens 7. The above reflected beam is converted into an electric signal by the photodetector 8.
FIG. 13 shows an example of a conventional optical head device wherein a diffraction element 9 comprising a holographic grating (hologram grating) is employed instead of the beam splitter 3. In this conventional optical head device, the laser beam source 1, the diffraction grating 2 are aligned with the collimating lens 4, the objective lens 5 and the like, while the photodetector 8 is placed by the side of the laser beam source 1 so that a beam reflected from the optical disk 6 is diffracted by the diffraction element 9 and guided to the photodetector 8.
In the conventional optical head device as shown in FIGS. 12 and 13, the laser beam is converged onto the optical disk 6 so as to have substantially the same width as that of a recording track 6a (the recording track 6a is hatched in FIG. 11 , for convenience of explanation) as indicated by mark S of FIG. 11 and the recording track 6a tracked by a tracking servo-mechanism (not shown in the drawings), thereby reading information recording on the recording track 6a in accordance with the beam reflected therefrom.
Since the recording track 6a formed on the optical disk 6 has a very narrow width of 1 to 2 .mu.m approx., not only is the laser beam to be irradiated onto the recording track 6a required to be converged into a small beam spot by the objective lens 5 having a high numerical aperture (NA), but also the beam spot must have a high luminous intensity. However, a drawback in the above method is realized in that an Airy ring indicated by mark S, is generated by a converged laser beam owing to the secondary maximum elements as well known by one skilled in the art. If the Airy ring extends to the adjacent recording tracks 6a, crosstalk will occur at the time of reading information stored on the optical disk.
An approach to solve the above problem is that as shown in FIG. 14(a), a grating face 2a of the diffraction grating 2 and a grating face 2b opposite thereto, which are shown in FIGS. 12 and 13, are respectively provided with a filter 10 mounted thereon. The filter 10 has a light transmitting section 10a the width of which is narrower than that of the laser beam to be transmitted therethrough. As shown in FIG. 14(b), the transmission factors of the zero other diffracted beam in the vicinity of both ends of the diffraction grating, (the ends set in a direction corresponding to the radial direction of the optical disk 6), are controlled with filter 10. These portions where the transmission factor of 0th order diffracted beam is controlled, are hatched in FIG. 14. This construction reduces the occurrence of crosstalk at the time of reading information recorded on the recording track 6a.
Such a filter 10 is generally produced by a metal vapor deposition. This production of the filter 10 by the vapor deposition is a complex manufacturing process as well as causing a high cost of production. Since an optical head device usually requires a large number of components; and therefore, the addition of the filter 10 further increases the number of components resulting in making the structure of the device complicated all the more.